When polyurethane moldings are made, conventionally for each shot a plurality of components (isocyanate, polyol, propellant, additives, etc.) are metered into a mixing chamber, mixed there and fed to a mold. For this mixing task, multiple component mixing heads are used, enabling the various components to be mixed at the same time.
Multiple component mixing heads of this kind conventionally have a cylindrical mixing chamber which is closed at one end and into which the various components are injected by way of inlet nozzles arranged on the periphery. In this arrangement, the inlet nozzles are conventionally arranged on a circular line so that the component jets entering the mixing chamber meet and are mixed. Generally, a cylindrical outlet channel closed at one end is arranged downstream of the mixing chamber in the outgoing direction, and the mixture produced exits the mixing head through this channel. Conventionally, the mixing chamber and the outlet channel are arranged at an angle of 90° with respect to one another so that the mixture is diverted as it leaves the mixing chamber and so kinetic energy is dissipated.
In this arrangement, these mixing heads furthermore have a control slide means which is arranged in the mixing chamber movably in the axial direction and which, in the retracted or rear position, frees the inlet openings so that the components can come out of the inlet nozzles and into the mixing chamber and meet (injection position), and which, in the extended or front position, connects the inlet nozzles to grooves which are made in the control slide means and through which the components flow to the respective outlets arranged on the cylindrical wall surface, through which they can be guided back into their reserve containers again (return position, recirculation).
The mixing heads typically have a cleaning piston or cleaning slide means which is arranged in the outlet channel movably in the axial position. The cleaning piston allows residues of the components which are still in the outlet channel after the end of injection to be removed so that the outlet channel is prevented from clogging.
Because these mixing heads have both a control slide means and a cleaning slide means, these mixing heads are also called two-slide mixing heads. Examples of mixing heads of this kind are described in DE-A-23 27 269, DE-C-29 07 938 and DE-A-30 40 922.
Hitherto, two-component, four-component and six-component mixing heads have been usual, with a separate groove in the control slide means being associated with each component.
In this arrangement, in particular with six-component mixing heads, the following problem has to be addressed: the diameter of the outlet channel is more or less predetermined because an average outgoing velocity of approximately 0.5 to 1 m/s, depending on the viscosity of the mixture, must be maintained as a minimum. Otherwise, the outflowing reaction mixture may not completely fill the cross-section of the outlet channel, which results in disruptive turbulence and in air entering the mixture.
With the known types of construction of multiple component mixing heads having two slide means, the diameter of the mixing chamber is always smaller than the diameter of the outlet channel, as it has been assumed up until now that the mixing chamber must be particularly small in high-pressure mixing because a small spacing between the openings of the inlet nozzles for the various components improves the atomizing effect of the component jets and hence also the extent of mixing (DE-C-29 07 938).
However, as a result of the very small diameters of the mixing chambers as known in the art, and in particular with small outgoing quantities, a structural problem arises: it is then the case that for example six control grooves have to be arranged spatially parallel next to one another on the limited periphery of the control slide means, while still leaving space for the seal between the control grooves.
As a consequence of this problem, in the case of the known constructions the grooves become too narrow and the loss in pressure during recirculation becomes too great. The solution to this dilemma has hitherto been to allow only a partial flow of the components through the grooves during recirculation, and to have most of the component flows guided past the mixing head by way of bypass lines, in which case, once the control slide means has been moved around into the injection position, they have also to be deflected. However, this is a complicated and relatively expensive solution born out of need.
A further problem of the known constructions is part of a basic problem of high-pressure mixing, in particular when small quantities are ejected.
Taking as a starting point laminar jets from the nozzles, it is necessary to achieve good mixing of the components which is as complete as possible in as short a time as possible, without the aid of additional fittings such as those installed in a static mixer. In ideal, undisrupted and hence laminar flow conditions in the mixing chamber, this is in principle achievable only with difficulty, as a diffuse transport of the material cannot make a more than negligible contribution to the mixing because of the short dwell times.
In a flow of this kind, and as a function of the jet pulses of the component jets, a theoretical impact point is formed around which a pressure gradient field is produced in the manner of a source point. This ensures that the individual flow paths to a greater or lesser extent approach this impact point as a function of their remaining velocity, with the dynamic kinetic energy being substantially converted to pressure energy and in addition to a certain extent being dissipated into heat as a result of the viscosity forces. This pressure energy is converted to kinetic energy again to give a smaller pressure. In this case, however, where there is an ideal undisrupted flow, a more or less stable interface is produced between the components, which in practice is not penetrated. This means that the components remain substantially unmixed.